1. Field Of The Invention
The present invention relates to video image enhancement. More particularly, the present invention relates to digital electronic noise-reduction techniques for high-quality video image improvement.
2. The Prior Art
It is known in the prior art to use median filters and temporal-recursive filters as effective methods for video image noise reduction. These two filtering methods may be used individually, or in combination, for better overall performance.
Median filtering is also known as rank-value filtering or rank-order filtering. By any name it is a well-known image-processing technique that combines pixels in a non-linear manner, and is particularly effective against impulsive noise and film grain and dirt when a three-dimensional pixel cluster is employed. Because median filters operate on discrete pixel values, the video must be in digitized form by nature.
Recursive filtering combines pixels spaced by exactly one video frame in an algebraic manner through controlled feedback, and is effective at reducing random noise by decreasing the temporal resolution in the noisy areas of the image while always preserving the horizontal and vertical resolution. Recursive filtering is not restricted to digital video images by nature, although providing an exact one-frame recursion delay is difficult by any other means. Median filtering coupled with recursive filtering gives better overall noise-reduction performance than either method when used independently.
The use of median and recursive filters for image processing has been reported in the literature. U.S. Pat. No. 4,058,836 to Drewery et al, teaches noise reduction by means of a recursive filter controlled by a motion detector. G. Wischermann, "Median Filtering of Video Signals--A Powerful Alternative", SMPTE Journal July 1991, discloses the benefit of the use of median filters in video images. A. Christopher et al., "A VLSI Median Filter for Impulse Noise Elimination in Composite or Component TV Signals", IEEE Transactions on Consumer Electronics, Vol. 34, No. 1, Feb. 1988, discloses the use of a pixel-replacement threshold to reduce median filter artifacts. U.S. Pat. No. 4,928,258 to May, teaches the use of a median filter in two and three dimensions with multiple weighted inputs, also known as multiple-input counting. British Patent Application No. GB 2 139 039 A to Storey, teaches electrical means for detecting the presence of film dirt in video signals.
In addition, at least one commercially available noise reduction system employs both median and recursive filtering. Broadcast Television Systems, Inc. of Salt Lake City Utah offers a model MNR9 Median noise reducer which employs selectable pixel clustering. The BTS product is prone to strong motion artifacts.
The state of the art in video image improvement using either median filtering or recursive filtering fails short of providing sufficient performance, closely related to noise reduction effectiveness, with acceptable motion artifacts and resolution loss in pictures with high motion content. In order of discovery, the Drewery teaching of recursive-only noise reduction is very fundamental, but the system performance reaches its limit for images with average signal-to-noise ratio (SNR) before producing noticeable motion artifacts. The concept of video random and impulsive noise reduction by means of a median filter is shown by Wischermann. While in this application the performance is quite good with still pictures, small motion in the picture produces motion artifacts and loss of video resolution. Christopher et al. implemented a manual pixel-replacement threshold logic at the output of a two-dimensional median filter in an attempt to minimize the blurring artifacts, but use of this threshold alone compromises the median filter effectiveness. May's invention is not geared toward high-quality video images, hence motion artifacts and picture resolution loss are more acceptable in his application. The Storey disclosure represents the state of the art in motion detection, although there is no suggestion to employ it in combination with a median filter.
It is an object of tile invention to provide an apparatus and method for improving digital video images by removing noise and film grain and dirt through the use of digital electronic filtering, while creating a minimum of filtering artifacts.